Subscriber line interface circuits (SLICs) re employed by telecommunication service providers to interface a wireline pair with subscriber (voice - data) communication equipment. In order to be interfaced with different types of telecommunication circuits, including (single supply-based) low voltage circuits that provide digital codec functionality, the transmission channels of the SLIC must conform with a very demanding set of performance requirements, such a accuracy, linearity, low noise, filtering, insensitivity to common mode signals, low power consumption, and ease of impedance matching programmability. In a typical application, the wireline pair to which the SLIC is connected can vary from one installation to another, and may have a significant length (e.g., on the order of multiple miles), transporting both substantial DC voltages, as well as AC signals (e.g., voice and/or ringing). As a result, it has been difficult to realize a SLIC implementation having ‘universal’ use in both legacy and state o the art installations.
Advantageously, this problem is successfully addressed by the SLIC architecture disclosed in the '976 application, referenced above and diagrammatically illustrated in FIG. 1. As shown therein, the SLIC of the '976 application is partitioned into a high voltage analog section 100, that drives tip and ring conductors 11, 12 of a subscriber loop pair 10, and a mixed signal (low voltage and digital signal processing) section 200, which monitors and controls the operation of the high voltage analog section. High voltage analog section 100 is comprised of an integrated arrangement of functional analog signal blocks, and is interfaced with a DSP codec subsection 200C and a supervisory microcontroller subsection 200S of the mixed signal section 200. The high voltage section performs analog (e.g., voice, ringing) signal processing and interface functions of a conventional SLIC, based on control inputs and programmed parameters of the mixed signal section.
Included within the high voltage analog section 100 is a receive input unit 110, which interfaces and conditions voice signals and associated ancillary signals, such as metering tones, supplied from the DSP codec subsection 200C of the mixed signal section 200. In order to perform (codec-)compatible, differential mode signal processing of the signals it interfaces between the codec and the wireline pair, the receive input unit 110 is supplied with a reference voltage REF sourced from the codec. The reference voltage REF is selected in accordance with the available voltage parameters of the codec circuit, and typically has a value midway between the single supply voltage (Vcc) and ground. For reduced voltage circuit applications, such as those operating at value on the order of three volts, the reference voltage REF may correspond to a voltage on the order of 1.5 VDC (midway between Vcc and ground).
The receive input unit 110 is implemented as a voltage-sense, current-feed circuit, to which voice signals from the mixed signal section's codec 200C are applied. A sense resistor 111 is coupled to a voltage reference port 112, to which the reference voltage REF is supplied from the codec. In response to a voice representative voltage signal between the voice signal receiving (VRX) port 113 and the reference voltage port 112, the sense resistor 111 produces a received current signal irx representative of the voice signal. Complementary polarity copies of the received current signal irx are regenerated by a pair of (tip and ring associated) current mirrors 114 and 115, and applied over lines 117 and 118 to respective tip and ring amplifiers 140T and 140R of a dual mode tip and ring amplifier unit 140.
The output of the tip amplifier 140T is coupled to the tip conductor 11, while the output of ring amplifier 140R is coupled to the ring conductor 12 of the wireline pair 10. In addition, the tip amplifier output is coupled to first input 131 of a sense amplifier (SA) 130, and the ring amplifier output is coupled to a second input 132 of sense amplifier 130. The output path of the sense amplifier 130 from an output port thereof includes a series resistor 134 coupled to an output terminal 135.
The output terminal 135 may be coupled through a capacitor CH to the inverting (−) input 151 of a auxiliary external operational amplifier 150, the non-inverting (+) input 152 of which is coupled to the reference voltage REF supplied by the codec. The output 153 of the auxiliary amplifier is coupled to the codec, and may be fed back to an analog feedback monitor (AFM) port 119 of the receive input unit 110. An auxiliary sense resistor 120 is coupled between the voltage reference port 112 and AFM port 119. The AFM port provides the ability to close a loop from the output of the sense amplifier 130 through the auxiliary amplifier 150, in order to synthesize the output impedance of the tip/ring amplifiers.
The parametric values of the sense amplifier resistor 134 and the sense resistors 141T and 141R of the tip and ring amplifiers 140T/140R are defined to effectively track one another with a precise ratio, in the sense of the output transfer function of the tip and ring amplifiers, as coupled to the sense amplifier's voltage detector circuitry. With voltages across the tip and ring sense resistors 141T/141R of the tip/ring amplifiers coupled in complementary-polarity fashion to the voltage detectors of the sense amplifier 130, the sense amplifier's output port 135 will provide a voice signal summation output for differential mode voice signals, whereas common mode signals will mutually cancel.
During AC (voice signal) transmission, the auxiliary amplifier 150 between the sense amplifier output port 135 and the AFM port 119 provides feedback necessary to perform the impedance matching function. This converts the output current through capacitor CH to a voltage, which is fed to the AFM terminal 119 and is a prescribed level (e.g., −6 dB) below the VRX signal, to realize a defined gain for the receive input to tip and ring. In this mode, the tip/ring amplifier unit 140 converts the received voice signal voltage into a differential mode signal at the tip/ring interface. The output of the sense amplifier 130 at port 135 thereby provides the codec with a very precise current proportional to the AC loop current. This current is converted to a voltage, amplified by amplifier 150 and fed to the codec for processing, being injected back into the receive path to produced a prescribed gain from the injection point to the wireline interface.
Unfortunately, the device (here a codec) that sources the reference voltage REF for defining the AC signal reference voltage baseline of the receiver typically contains high frequency switching transients produced within the device, and thereby constitutes an unwanted noise source to the SLIC. Although this is not a problem for the differential mode receiver, it represents a significant impairment to the ability of the auxiliary amplifier 150 feeding the codec to deliver an output voltage precisely proportional to only the AC loop current. When the reference voltage REF is noisy, then that noise will propagate through (and be amplified by) the auxiliary amplifier, and will appear at the input to the codec.